The slime mold stores a record of where it's been using, well, slime.

Is it possible to know where you've been when you don't have a brain? Depending on your definition of "know," the answer may be yes. Researchers have shown that the slime mold, an organism without anything that resembles a nervous system (or, for that matter, individual cells), is capable of impressive feats of navigation. It can even link food sources in optimally spaced networks. Now, researchers have shown it's capable of filling its environment with indications of where it has already searched for food, allowing it to "remember" its past efforts and focus its attention on routes it hasn't explored.

And it does this all using, as the authors put it, "a thick mat of nonliving, translucent, extracellular slime." As you might expect, given the name.

Slime molds are odd creatures: organisms that have a nucleus and complex cells, but are evolutionarily distant from the multicellular animals and plants. When food is plentiful, they exist as single-celled, amoeba-like creatures that forage on the food. But once starvation sets in, the cells send out a signal that causes them to aggregate and fuse. This creates an organism that's visible to the naked eye and all a single cell, but filled with nuclei containing the genomes of many formerly individual cells. That turns out to be advantageous, because this collective can move more efficiently, and go about foraging for food. In the course of this foraging, the organism leaves behind a trail of slime.

In the course of studying the slime mold, some researchers noticed that the slime mold would avoid any areas covered in slime. So they decided to quantify that. They set up two equal food sources at both ends of a Y-shaped container, and put some slime mold at the base. One of the arms had plain media for the organism to crawl across; the other coated the surface of the media with slime. The results were dramatic: in 39 of the 40 tests they ran, the slime mold avoided the arm that was pre-slimed.

Based on this finding, the authors hypothesized that the mold "uses its [the slime's] presence as an externalized spatial memory system to recognize and avoid areas it has already explored." So, they came up with a test that, in their words, "challenged our slime mold."

They created a three-sided box, and placed it with the open end facing a slime mold. Behind the box, the authors placed a tasty treat of glucose, which could slowly diffuse through the box. But the slime mold was unable to pass through the walls of the box, and thus couldn't take a straight path towards the food source. To find it, it would have to explore the interior of the box, give up, and start looking for a way around it.

Within a few days, 96 percent of them had. But, when the authors covered the entire setup in slime so the mold had no way of telling what areas it had already visited, only about a third of the organisms succeeded in making their way to the food.

So, the authors conclude, the slime isn't just the mold's calling card. Instead, it's a way of marking the environment so that the organism can sense where it's been, and not expend effort on searches that won't pay off.

Although the situation isn't an exact parallel, the authors make a comparison to the pheromone trails used by ants. (This comparison may have come about because two of the authors work in the Behaviour and Genetics of Social Insects Laboratory at the University of Sydney.) No individual ants have to remember where a food source is. Instead, by laying down a trail of scent molecules, workers that find food leave a trail in the environment the ants can collectively use. In effect, the colony has a memory that's stored in the environment.

In this case, the slime mold does the opposite, in that it avoids the areas of its environment that are chemically marked. The other big difference, of course—the slime mold doesn't have a brain at all. Still, the general principles of placing a memory in the environment are the same.

This raises interesting questions about the nature of memory and of "self". If I write a post-it note reminding me that there's no milk in the fridge and that I need to buy some, is that part of my memory, or something external to me?

The next logical step in the experiment is to stick the substance in a toaster and play many different genres of music, carefully observing if the toaster decides to dance, stand pat, or try to kill you. This could be an incredible discovery.

This raises interesting questions about the nature of memory and of "self". If I write a post-it note reminding me that there's no milk in the fridge and that I need to buy some, is that part of my memory, or something external to me?

The post-it notes are external. Your memory of the fact that there is no milk and putting up the post-it notes are part of your memory.

One key point to understand is that the largest change in the process of evolution beyond the orginal bactieria like organism was the emergence of the Eukaryotic cell. Tom Cavalier-Smith who spoke at the Rockefeller conference that Dr. Timmer covered a few years ago has been a particularly strong proponent of this reality. His almost certainly correct thesis is that the function of phagocyosis was central to a large number of major innovations that marked the emergence of the animal cell. Among the major features of these innovations is the cytoskeleton composed in part of the same proteins that human muscles use for all human behavior.There appear to be several videos of Physarum movement out on the .web. The one entitled Tokyo rail network designed by Physarum plasmodia particularly interested me because the behavior of the plasmodium reminds me strongly of the behavior I studied in Physarum amoebae over thirty years ago. Using time lapse movies and some genetics I was able to describe some significant social behavior among the ameobae that appeared to be an organized group effort to systematically consume available food. The behavior varied between cases where the amoebae were fed live or formalin killed bacteria and also by pH. It was also possible to isolate mutants with qualitative behavioral changes.At the time I proposed that the results I observed were due to social behavior among the amoebae and that this kind of social behavior had lead to the evolution of the plasmodia. During part of the time I did this work I worked at a cancer research institute and was supported by a cancer foundation post doctoral fellowship. So I tried to seel the idea that metastaisis of cancer cells could largely be a function of a breakdown in cell behavior. Generally my ideas were regarded as somewhat wild. But the head of the cancer research institute did like my idea about behavior as a factor in metastatic cancer cells.Today these ideas seem to be more or less conventional wisdom. For a mind blowing web site with an encyclopedic collection of information about cell signalling mechanisms see Michael Berridge's web site titled Cell Signalling Biology

Indeed, and it apparently has a very finely tuned "efficiency assessment" system. Japanese researchers put food sources where the train stations in Tokyo are. Then they watched as a brainless organism replicated hundreds of hours of work done by Japanese engineers. In a day.

I'm amazed at the degree of adaptability and ingenuity of life, even the lower forms. Shows how much power hundreds of millions of years of evolution places in the hands of a seemingly dumb and "low life" organism.

This is why life is awesome. Slime molds are a particularly great exploration of "weird stuff". Gotta love the things.

Agreed: there is an obvious mechanism here that has come into being, strong enough that our poor brains have to anthropomorphize it as “memory.“ Whatever floats your boat; memory is amazing and this reaction that has evolved is also.

Stuff like this reminds you just how strange to us life elsewhere must look. If we even realised it was life if we ever got a chance to see it.

Although sometimes I wonder if intelligent life can only really exist in a form we'd be more used to and while it might look a little strange, it's something we'd be used to (brain, eyes, limbs of some sort).

Interesting. Is that kind of cellular fusion (for lack of a better term) unique to slime molds in particular, or are other species of mold (or for that matter, any other single-celled creature) capable of that?

I might be oversimplifying things, but is this 'external memory', or just an inherent tendency to avoid slime mold secretions? If I am searching for food, I won't go looking for it in a room filled with human feces and urine, either. Does that mean I'm using 'external memories'?

If I am searching for food, I won't go looking for it in a room filled with human feces and urine, either.

See how well that works? If the room is full of human feces and urine then everything you would want to eat or drink has probably already been consumed. Revulsion of our own excrement keeps us from wasting time in places that are already depleted.

See how well that works? If the room is full of human feces and urine then everything you would want to eat or drink has probably already been consumed. Revulsion of our own excrement keeps us from wasting time in places that are already depleted.

I'll remember that next time I am out on a date and need to use the facilities. "Excuse me, dear; I need to go lay down a memory."

But, getting back to my earlier point, just because the slime molds avoid their secretions doesn't mean the reason they lay the secretions down is to signify the area has been depleted of food. I think the researchers (or their PR people) are being pretty heavy handed to call these 'external memories'.

I've come realize that when things like this crop up, it's more like the researchers are proposing that "external memory" is a useful way to think of the situation, which might stimulate new insights, rather than an absolute and final description. Instead of "Aha! Slime molds do all their memorizing on the outside!" it's "What if this had similar functions to our memories and internal maps, in that it allows X and Y behaviors that could be beneficial?"

Is that kind of cellular fusion (for lack of a better term) unique to slime molds in particular, or are other species of mold (or for that matter, any other single-celled creature) capable of that?

To be clear, as the xkcd comics hints at, slime molds is a large and disparate group which may or may not be polyphyletic as such.

One major trait is plasmodial (multiple diploid nucleus) when feeding as here. Another major trait is modal multicellularity (coming together and diversifying for procreation) when starving as the model organism Dictyostelium discoideum [ http://en.wikipedia.org/wiki/Dictyostelium_discoideum ].

One key point to understand is that the largest change in the process of evolution beyond the orginal bactieria like organism was the emergence of the Eukaryotic cell. Tom Cavalier-Smith who spoke at the Rockefeller conference that Dr. Timmer covered a few years ago has been a particularly strong proponent of this reality. His almost certainly correct thesis is that the function of phagocyosis was central to a large number of major innovations that marked the emergence of the animal cell.

The problem I have with Cavalier-Smith's models is that they are wildly massive and make little solid prediction out of all that, akin to building air castles. His papers are 80+ pages tomes with many hundreds of references, and his strategy may be to tire peer reviewers out. (Not kidding, nor am I alone to make that claim as I remember it.)

He may certainly get some things correct that way. Phagocytosis first is not necessary though. Bacteria-bacteria endosymbionts have been found, at least within insects. And the type of endosymbiosis that made for the mitochondria energy plant is that made eukaryotes special, according to Lane's energy theory.

Megaviruses that split off so early that they predict a 4th viral domain* contain endocytosis proteins, which would likely make Cavalier-Smith correct in the sense that the two other domains lost that. (The gene list is in the Megavirus paper describing the find, IIRC.)

Interesting. Is that kind of cellular fusion (for lack of a better term) unique to slime molds in particular, or are other species of mold (or for that matter, any other single-celled creature) capable of that?

Human muscle cells are poly-nucleated, and this is the result of several cells fusing.

They forgot to answer the most important question... What does slime mold slime taste like?

They didn't forget, it's... they can't.

It's better not to ask them again. :-)

Quote:

Maybe slime mold have a primitive sense of taste and their avoidance of their own slime is a primitive version of self-loathing. Complex behavior through chemistry!

The exact opposite of human if that's the case. A person wouldn't mind to cleaned-up his own poop after but feels differently to poop from other person. "Oh man that stink!" "Your poop smell awful mine is hmmm.. good"

I would suggest the researchers tried placed slime left over from other slime mold, the result may be not the same.